Ab Initio Study of Ion-Pair States of Halogen Molecules

Abstract

The ion-pair states of IBr, ICl, and BrCl molecules correlating to the limits of dissociation X+(3P2,1,0, 1D2) + Y–(1S0) are studied using a complete active space self-consistent field (CASSCF), allowing for dynamic electronic correlations and spin–orbit interaction. Using experimental results, the calculated values of the equilibrium energy of the X+Y−(3P2) states correlating to the lowest ion state X+(3P2) are grouped in the range ΔTe ∼ 100 cm−1, with the error of their relative location also on the order of 100 cm−1. For most states, the difference between the experimental and calculated values of the equilibrium internuclear distance does not exceed $$R_{{\text{e}}}^{{\exp }}$$ – $$R_{{\text{e}}}^{{{\text{calc}}}}$$ = 0.02 A. A comparative analysis of structural features of the ion-pair states of homonuclear and heteronuclear halogen molecules is performed using results from recent ab initio calculations for the I2 molecule. We also discuss some issues concerning ion-pair states X−Y+ with inverted charge localization, which correlate to the limits of dissociation X– + Y+ and are coupled with the X+Y− states by two-electron transitions. It is noted that the structures with two stable localizations of two electrons, X+‒D‒Y− and X−‒D‒Y+, where X, Y are electron affinity centers (atoms, molecules, clusters) and D is a dielectric spacer, could be of interest for microelectronics.